Designers, manufacturers and users of electronic computers and computing systems require reliable and efficient equipment for storage and retrieval of information in digital form. Conventional storage systems, such as magnetic disk drives, are typically utilized for this purpose and are well known in the art. However, the amount of information that is digitally stored continually increases, and designers and manufacturers of magnetic recording media work to increase the storage capacity of magnetic disks.
In conventional magnetic disk data/information storage, the data/information is stored in a continuous magnetic thin film overlying a substantially rigid, non-magnetic disk. Each bit of data/information is stored by magnetizing a small area of the thin magnetic film using a magnetic transducer (write head) that provides a sufficiently strong magnetic field to effect a selected alignment of the small area (magnetic grain) of the film. The magnetic moment, area and location of the small area comprise a bit of binary information which must be precisely defined in order to allow a magnetic read head to retrieve the stored data/information.
Such conventional magnetic disk storage media incur several drawbacks and disadvantages which adversely affect realization of high areal density data/information storage, as follows:
(1) there are an infinite number of possibilities for the magnetic moments of the continuous magnetic film and, as a consequence, the write head must be able to write very precisely in order to precisely define, without error, the magnetic moment, location and area of each bit on the magnetic film;
(2) since the continuous film tends to link exchange and magnetostatic interaction between neighboring magnetic bits, when the bits are very close, writing of one bit can result in writing of neighboring bits because of the exchange and magnetostatic interaction, causing errors in reading;
(3) the absence of physical boundaries between many bits of the continuous magnetic film cause the writing and reading process to occur in a “blind” fashion, i.e., the location of each bit is determined by calculating the movements of the disk and the read or write heads instead of physically sensing the actual bit location;
(4) the boundaries between adjacent pairs of bits tend to be ragged in continuous magnetic films, resulting in noise generation during reading; and
(5) the requirement for increased areal recording density has necessitated a corresponding decrease in recording bit size or area. Consequently, recording bit sizes of continuous film media have become extremely minute, e.g., on the order of nanometers (nm). In order to obtain a sufficient output signal from such minute bits, the saturation magnetization (Ms) and thickness of the film must be as large as possible. However, the magnetization quantity of such minute bits is extremely small, resulting in a loss of stored information due to magnetization reversal by “thermal fluctuation”, also known as the “superparamagnetic effect”.
Regarding item (5) above, it is further noted that for longitudinal type continuous magnetic media, wherein the magnetic easy axis is oriented parallel to the film plane (i.e., surface), magnetization reversal by the superparamagnetic effect may occur even with relatively large magnetic particles or grains, thereby limiting increase in areal recording density to levels necessitated by current and future computer-related applications. On the other hand, for perpendicular type continuous magnetic media, wherein the magnetic easy axis is oriented perpendicular to the film plane (i.e., surface), growth of the magnetic particles or grains in the film thickness direction increases the volume of magnetization of the particles or grains while maintaining a small cross-sectional area (as measured in the film plane). As a consequence, onset of the superparamagnetic effect can be suppressed for very small particles or grains of minute width. However, further decrease in grain width in perpendicular media necessitated by increasing requirements for areal recording density will inevitably result in onset of the superparamagnetic effect even for such type media.
The superparamagnetic effect is a major limiting factor in increasing the areal recording density of continuous film magnetic recording media. Superparamagnetism results from thermal excitations that perturb the magnetization of grains in a ferromagnetic material, resulting in unstable magnetization. As the grain size of magnetic media is reduced to achieve higher areal recording density, the superparamagnetic instabilities become more problematic. The superparamagnetic effect is most evident when the grain volume V is sufficiently small such that the inequality Kμ V/kBT>40 cannot be maintained, where Kμ is the magnetic crystalline anisotropy energy density of the material, kB is Boltzmann's constant, and T is the absolute temperature. When this inequality is not satisfied, thermal energy demagnetizes the individual magnetic grains and the stored data bits are no longer stable. Consequently, as the magnetic grain size is decreased in order to increase the areal recording density, a threshold is reached for a given Kμ and temperature T such that stable data storage is no longer possible.
So-called “patterned” or “bit-patterned” magnetic media (the former expression will generally be utilized herein) have been proposed as a means for overcoming the above-described problems associated with continuous magnetic media, e.g., as disclosed in U.S. Pat. Nos. 5,820,769 and 5,956,216, the entire disclosures of which are incorporated herein by reference. In this context, the term “patterned” media refers to magnetic data/information storage and retrieval media wherein a plurality of discrete, independent regions of magnetic material form discrete, independent magnetic elements that function as recording bits are formed on a non-magnetic substrate. Since the regions of ferromagnetic material comprising the magnetic elements are independent of each other, mutual interference between neighboring elements can be minimized. As a consequence, patterned magnetic media are advantageous vis-a-vis continuous magnetic media in reducing recording losses and noises arising from neighboring magnetic bits. In addition, patterning of the magnetic layer advantageously increases resistance to domain wall movement, i.e., enhances domain wall pinning, resulting in improved magnetic performance characteristics.
Generally, each magnetic element has the same size and shape, and is composed of the same magnetic material as the other elements. The elements are arranged in a regular pattern over the substrate surface, with each element having a small size and desired magnetic anisotropy, so that, in the absence of an externally applied magnetic field, the magnetic moments of each discrete magnetic element will be aligned along the same magnetic easy axis. Stated differently, the magnetic moment of each discrete magnetic element has only two states: the same in magnitude but aligned in opposite directions. Each discrete magnetic element forms a single magnetic domain and the size, area and location of each element or domain is determined during the fabrication process.
During writing operation of such patterned media, the direction of the magnetic moment of the single magnetic domain or element is flipped along the easy axis, and during reading operation, the direction of the single magnetic domain or element is sensed. The direction of the magnetic easy axis of each single magnetic domain or element can be parallel or perpendicular to the surface of the domain or element, corresponding to conventional continuous longitudinal and perpendicular media, respectively. Stated differently, the nature (i.e., type) of the magnetic recording layer of the magnetic domains or elements is not critical in patterned media, and may, for example, be selected from among longitudinal, perpendicular, laminated, anti-ferromagnetically coupled (AFC), granular, superlattice, types.
Patterned media in disk form offer a number of advantages relative to conventional disk media. Specifically, the writing process is greatly simplified, resulting in much lower noise and lower error rate, thereby allowing much higher areal recording density. In patterned disk media, the writing process does not define the location, shape and magnetization value of a bit, but merely flips the magnetization orientation of a patterned single element or domain. Writing of data can be essentially perfect, even when the transducer head deviates slightly from the intended domain or element location and partially overlaps neighboring domains or elements, as long as only the magnetization direction of the intended domain or element is flipped. By contrast, in conventional magnetic disk media, the writing process must define the location, shape and magnetization of a bit. Therefore, with such conventional disk media, if the transducer head deviates from the intended location, the head will write to part of the intended bit and to part of the neighboring bits. Another advantage of patterned media is that crosstalk between neighboring domains or elements is reduced relative to conventional media, whereby areal recording density is increased. Each individual magnetic element or domain of a patterned medium can be tracked individually, and reading is less jittery than in conventional disks.
As indicated above, the escalating requirements for increased data/information storage capacity necessitate development of magnetic media with ultra-high areal recording density. In order to achieve a recording density of about 1 Tbit/in2 with patterned media, a nanostructure array of magnetic elements, domains, or “dots” (as with circular columnar-shaped elements or bits) with a period of about 25 nm over a full-patterned 2.5″ diameter disk surface is required. While fabrication methods supporting element or dot densities up to about 300 Gbit/in2 have been demonstrated, large area ultra-high density magnetic element patterns necessary for Tbit/in2 recording densities are currently unavailable or not achievable in a cost effective manner.
The use of multiple level (multilevel) magnetic storage media has been proposed as a means for increasing the areal storage density of continuous media (see, e.g., U.S. Pat. No. 5,583,727, the entire disclosure of which is incorporated herein by reference) and bit patterned media (see, e.g., M. Albrecht, et al., J. Appl. Phys. 97, 103910 (2005) or U.S. Pat. Nos. 6,865,044 B1, 6,882,488 B1, 6,906,879 B1, 6,947,235 B2, the entire disclosures of which are incorporated herein by reference). Multilevel patterned media offer an advantage over single level patterned media in that an increase in areal recording density is possible without further increase in element density, thereby facilitating manufacture. A disadvantage inherent with practical use of the multilevel continuous film media of e.g., U.S. Pat. No. 5,583,727, is that the number of magnetic grains, and hence the read signal and media noise, are divided into the multiple levels, thereby degrading the signal-to-media noise ratio (SMNR).
On the other hand, in multilevel patterned media comprising elements with a stacked plurality of magnetic cells, each cell including a magnetic recording layer is magnetically decoupled from overlying or underlying cells by non-magnetic spacer layers. Therefore, in the case of patterned media comprising a stack of cells with perpendicular magnetic recording layers of different coercivity, each cell of an element can have a magnetization moment or direction in one of two distinct directions, i.e., into or out of the plane of the magnetic layer of the cell, and this magnetization direction is independent of the magnetization direction of the other cells of that element. As a consequence, multiple magnetic states can be recorded in each element. In contrast with multilevel continuous film media, because each cell of the element constitutes a single magnetic domain, there is no increase in noise due to the multiple magnetic cells or levels. The plurality of magnetic cells or levels stacked in the bit or element generates a corresponding plurality of different readback signal levels, whereby the areal recording density is increased.
However, a disadvantage of the proposed scheme for utilizing multilevel patterned media arises from the requirement that each level be addressed individually. Stated differently, multiple passes of the write head over the media are necessary for writing data to each level. However, it is evident that, for such a write procedure, the data rate disadvantageously incurs a substantial decrease.
In view of the foregoing disadvantage/drawback associated with the use of multilevel patterned media, which disadvantage/drawback constitutes an impediment to implementation of multilevel media technology and methodology in ultra-high areal recording density applications, there exists a need for improved methodology that eliminates, or at least mitigates, the existing requirement for multi-pass writing of multilevel patterned media.